1. Field of the Invention
The present invention relates to a lithographic apparatus and a method for manufacturing a device.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In the known scanner type lithographic apparatus, a reticle masking device is used to block any light which would be projected outside the actual pattern to be projected. Such reticle masking device may have the following benefits. The reticle masking device (1) removes the need for having a wide (i.e., expensive) light blocking border area, for instance of chrome, (2) blocks light that might otherwise leak through pin holes in the border area, (3) allows a selected portion of the full patterned area to be exposed, and (4) selectively blocks reticle alignment targets so that they are not printed on the wafer.
Generally, the reticle masking device of the known lithographic apparatus includes four independently movable REMA blades configured as two pairs. One pair of REMA blades has edges that are aligned parallel to the exposure scan axis. This first pair remains normally stationary during the exposure scan and delimits the width of the exposed field. A second pair of REMA blades has edges that are aligned orthogonal to the scan axis. This second pair moves synchronously with the reticle and delimits the dimension of the radiation beam which is to be projected on a patterning device and thus of the length of the exposed field. An example of the known reticle masking device is for instance disclosed in US 2005/0200830A1, the contents of which are herein incorporated by reference.
In the known lithographic apparatus, it is desirable to locate the REMA blades at a focus plane of the illumination system, i.e. an image plane conjugate to the patterning device, rather than near the patterning device itself. Because of a minimum distance between the patterning device, located in a focal plane, and the reticle masking device, the masking blades produce a relatively large half-shadow, resulting in a requirement for a large chrome border around the pattern on the reticle. Note that the blades are never imaged on the wafer. To further increase the throughput of a lithographic apparatus it has been proposed to place two patterning devices in a single patterning device support so that the patterns of each of the first and the second patterning device may be projected on the substrate in a single scanning movement. When in such method use is made of a reticle masking device to obtain the above described advantages for such device, the reticle masking device will be moved in the scanning direction through the radiation beam to provide a ‘scanning’ radiation beam. At the end of the scanning of the pattern of the first patterning device, the reticle masking device will have to be moved from one side of the radiation beam to the original position of the reticle masking device at the other side of the radiation beam, before the required scanning movement of the reticle masking device can be made for the second patterning device.
Since it is undesired to change the speed with which the patterning device support is moved and the distance between the first and the second patterning device is relatively short, there is only a very small time available to move the reticle masking device back to the original position.
A drawback of the present known lithographic apparatus is that in most cases the available time will not be sufficient to move the reticle masking device back to the original position to start a new scanning movement.